To rescue biodiversity, we need a better way to measure it

At first blush, the idea of biodiversity seems simple enough. It is essentially the variety of all life on Earth. But making sense of biodiversity in a way that can help us halt or even reverse its decline is anything but straightforward.

“People often use the word biodiversity just to mean any characteristic of life out there that we might care to protect,” says Mark Vellend, a biologist at the University of Sherbrooke in Quebec, Canada. “That’s not a definition I find useful in science because if it’s everything, it’s nothing.”

For biodiversity to be a valuable concept, he says, it needs to be a measure of biological variety. That way, we can not only assess where we are and where we are headed, but also how best to conserve the biodiversity we have left.

The problem is that variety itself comes in many forms, especially in biology. “You can’t just come up with a single number for biodiversity in the same way as you can for carbon,” says Andy Hector at the University of Oxford. “It’s way, way more complicated.”

We already have ways to measure biodiversity. That’s how we know it is in steep decline. They boil down to what biologists think of as dimensions of biodiversity. One of the most basic is species richness, which is simply the number of species in a given place at a given time. This has been used extensively and can sometimes be a useful proxy for other dimensions of biodiversity, says Hector.…